Biologically active substances such as found in serums: like enzymes; hormones; electrolytes and biologically active metabolites, are used widely in the diagnosis of diseases. They are used as reference standards for instrumental automated colorimetric analysis since they contain all or most of the components of the unknown to be analyzed. Once the diagnosing physician is aware of the patient's concentration of components, viz., differences versus normal mean ranges of concentration of such components, the diagnosis can be made more objectively. In their natural form, when separated from their normal biological environment, such biologically active substances are unstable and undergo undesirable changes under the influence of heat, enzyme action, hydrolysis and other influences causing undesirable molecular transformations therein. In the past, several methods of preservation have been utilized for such labile biological products.
One such procedure involves "freeze-drying" of the biological. The freeze-drying procedure essentially involves rapidly reducing the temperature of the aqueous-containing biological followed by dewatering it to a very substantial, if not total, extent at reduced pressures. The freeze-dried biologicals can be stored as a solid for varying lengths of time depending on their composition. Thus, for example such dehydrated control material as blood serum or blood plasma can be stored for 1 to 2 years when stored at 2.degree. to 8.degree. C.
At present dehydrated freeze-dried biological reference control compositions are reconstituted with distilled water shortly before their intended use.
Once reconstituted their shelf life is usually from three (3) to twenty-four (24) hours when stored in liquid form at 2.degree. to 8.degree. C. Length of stability depends on the nature of the biological material. Small biological molecules, e.g., electrolytes, will be among the most stable whereas large molecules, such as enzymes, are among the least stable. Thus acid phosphatase can decompose within the first two hours after reconstitution with water from the freeze-dried dehydrated state whereas sodium and potassium can be stable for several days.
Customarily the unused portion of freeze-dried reconstituted biological control compositions must be either refrozen to solid form or discarded at the end of each working day. The first method of extending storage life is both expensive (energy wise) because refreezing requires temperatures of -20.degree. to -30.degree. C. and time consuming because of the time required for rethawing. The second method is obviously wasteful and hence expensive. The process of this invention offers an inexpensive, convenient solution to the aforementioned problems.
The present invention extends the useful shelf (storage) life of reconstituted freeze-dried biological reference control compositions economically and permits them to be kept at 2.degree. to 8.degree. C. in the liquid state for periods of four to five weeks thus not only avoiding substantially the instability problems encountered with refrigerator storage (at 2.degree. to 8.degree. C.) of compositions reconstituted in accordance with the prior art but also avoiding the necessity of refreezing to solid form for storage (at -20.degree. C.) and rethawing for use. Significant improvement in the shelf life at 2.degree. to 8.degree. C. of freeze-dried reconstituted biological reference control compositions have been attained and instability arrested in such compositions containing uric acid, glucose, bilirubin, a variety of enzymes, e.g., alkaline phosphatase, etc. using the process improvement of this invention.